}

  public void testOnlyElementNull() {
    assertThat(Stream.<@Nullable Object>of((Object) null).collect(onlyElement())).isNull();
  }

  public void testOnlyElementMultiple() {
    IllegalArgumentException expected =
        assertThrows(IllegalArgumentException.class, () -> Stream.of(1, 2).collect(onlyElement()));
    assertThat(expected).hasMessageThat().contains("1, 2");
  }